Magnetic particles and process for producing the same

Abstract: Magnetic particles, useful as a colouring pigment and in a magnetic toner, comprise magnetite particles containing in the inside thereof elemental silicon in an amount of from 0.10 to 1.00 wt% (calculated as Si), and a hydrous coprecipitate of silica and alumina on said magnetite particle surfaces, and have an area-average diameter of from 0.05 to 1.0 µm.

Description: The present invention relates to magnetic particles and a process for producing the magnetic particles and more in detailed, the present invention relates to magnetic particles for a magnetic toner which have a high fluidity, a low oil absorption, an excellent charging stability and a good initial dispersibility, and a process for producing such magnetic particles. Hitherto, a development method of using as a developer composite particles prepared by mixing and dispersing magnetic particles such as magnetite particles in a resin without using a carrier, so-called one-component magnetic toner, has been widely known and used as one of the electrostatic latent image development methods. With the enhancement of the image qualities such as image density and gradient and the increase of printing speed of the copying machines in the recent years, more improvements of the properties of the magnetic toner used as a developer are demanded. That is, a magnetic toner is required to have an improved developing-stability so as to allow copying with a uniform image density, a high durability so as to enable clear and uniform copying even after repetition of copying, an excellent environmental stability so as to allow normal copying even under the low-temperature and low-humidity condition or high-temperature and high-humidity condition, as well as excellent fluidity and charging stability. Regarding the durability of magnetic toner, Japanese Patent Application Laid-Open (KOKAI) No. 4-162051 states: "Further, since the copying machines are trending toward higher printing speed, a toner is required to satisfy the high-level requirements for high resolution, high-speed development, high durability, etc. ···" As to the developing stability and fluidity of magnetic toner, Japanese Patent Application Laid-Open (KOKAI) No. 53-94932 describes: "Such high-resistance magnetic toner is poor in fluidity because of high resistance and tends to cause uneven development. That is, the high-resistance toner for PPC can retain the electric charges necessary for transfer, but since there remains a small amount of charges in the toner bottle or on the surfaces of the magnetic rolls even in the steps (other than the transfer step) where no charging is necessary, due to the frictional charging or mechanoelectret in the toner producing process, the agglomeration of particles tends to take place, thereby causing lowering of fluidity." and "Another object of the present invention is to provide a high-resistance toner for PPC having an improved fluidity to obtain high-quality indirect photocopies with no unevenness of development, hence with excellent resolution and gradient." Concerning the environmental stability, the fluidity and the charging stability of magnetic toner, Japanese Patent Application Laid-Open (KOKAI) No. 63-139367 states: "In the method using such a dry developer, in order to form a high-quality visible image, the developer is required to have a high fluidity and uniform charging properties, and for this reason, it has been generally practiced to add and mix fine silica particles with toner particles. However, since the silica particles are per se hydrophilic, the developer particles incorporated with the said silica particles tend to agglomerate with the aid of moisture in the air, resulting in reduced fluidity of the developer, and in certain cases, the charging efficiency of the developer may even be lowered due to moisture absorption of silica. ······ It is required that the good copying quality be maintained even when the copies are left under a high-humidity condition for a long time, and in this respect, the conventional hydrophobic silica particles were unsatisfactory." and "An object of the present invention is to provide a statically charged developer which is stable against changes of environmental factors such as high or low temperature and humidity, and able to maintain and show the good and excellent properties." There is a close relation between the properties of the magnetic toner and the properties of the magnetic particles dispersed in the magnetic toner. For improving the developing stability, the durability and the environmental stability of the magnetic toner, it is required that the magnetic particles used for the toner have a good miscibility with the resin and be minimized in oil absorption, as pointed out in Japanese Patent Application Laid-Open (KOKAI) No. 55-65406 which states: "Generally, the magnetic particles for such single-component magnetic toner are required to meet the following property requirements: ······ (VII) To have a good miscibility with resin. The toner particle size is usually not more than 10 µm, and the microscopic mixing degree in the toner is of much significance for the toner properties." As for the fluidity of magnetic toner, since it greatly depends on the surface condition of the magnetic particles exposed on the magnetic toner surfaces, it is strongly required that the magnetic particles are excellent in fluidity. The charging stability of magnetic toner is greatly dependent on distribution of the charge on the magnetic particles exposed on the magnetic toner particle surfaces, so that it is keenly required that the distribution of the charge on the magnetic particles be minimized. Further, since the fluidity and charging stability of magnetic toner are greatly dependent on the surface condition and uniformity of the charging of the magnetic particles as mentioned above, it is necessary to maintain the surface condition and charging stability of the magnetic particles in a favorable state throughout the magnetic toner production process. For this, it is emphatically required that the good initial dispersion be secured in kneading of the magnetic particles with resin. Various attempts have been made for improving the properties of the magnetic particles contained in resin so as to improve the properties of magnetic toner. For example, the following methods have been proposed: (1) A water-soluble silicate is added in the course of the reaction for producing the magnetite particles of various shapes by a wet process, in which an oxidation reaction is carried out by passing an oxygen-containing gas through a suspension containing colloidal ferrous hydroxide obtained from the reaction of an aqueous ferrous salt solution and an aqueous alkaline solution (Japanese Patent Publication (KOKOKU) No. 3-9045 and Japanese Patent Application Laid-Open (KOKAI) Nos. 3-131863 and 5-72801). For example, Japanese Patent Publication (KOKOKU) No. 3-9045 discloses "Spherical magnetite particles having a bulk density of 0.40 to 1.00 g/cm 3 , containing Si in an amount of 0.1 to 5.0 atom% based on Fe and having an excellent temperature stability" and "In a production of spherical magnetite particles which comprises passing an oxygen-containing gas through a ferrous salt reaction solution containing colloidal ferrous hydroxide obtained by reacting an aqueous ferrous salt solution and an alkali hydroxide of an amount of 0.80 to 0.99 equivalent to Fe 2+ in said ferrous salt solution, the magnetite particles production comprising the first stage in which a water-soluble sllicate is added in an amount of 0.1 to 5.0 atom% (calculated as Si) based on Fe, to the said alkali hydroxide or the said aqueous ferrous salt solution containing colloidal ferrous hydroxide, and then an oxygen-containing gas is passed through the resultant solution under heating at 70 to 100°C, thereby producing the spherical magnetite particles from said colloidal ferrous hydroxide, and the second stage in which an alkali hydroxide is added in an amount of not less than 1.00 equivalent to residual Fe 2+ and oxidation under heating is further carried out under the same conditions as in the first stage." (2) A silicon compound is contained in the inside of the spherical magnetite particles and also exposed at the surfaces thereof (Japanese Patent Application Laid-Open (KOKAI) No. 5-213620). Specifically, the above Japanese Patent Application Laid-Open (KOKAI) No. 5-213620 proposes "Magnetite particles having a silicon compound incorporated therein and exposed at the particle surfaces" and "A process for producing magnetite particles which comprises adding a silicon compound to a solution containing a ferrous salt as a main ingredient, mixing an alkali in an amount of 1.0 to 1.1 equivalent, carrying out an oxidation reaction while maintaining pH at 7 to 10, supplying deficient iron in an amount of 0.9 to 1.2 equivalent to the initial alkali in the course of reaction, and continuing the oxidation reaction at pH 6 to 10." (3) An a hydroxide of aluminum and/or an oxide of aluminum are deposited on the surfaces of the magnetite particles of various shapes produced by a wet process (Japanese Patent Application Laid-Open (KOKAI) Nos. 54-139544, 61-53660, 2-73367 and 4-162051). For example, Japanese Patent Application Laid-Open (KOKAI) No. 61-53660 proposes "A process for producing a magnetic developer which comprises mixing a magnetic powder with a binder resin and granulating the obtained mixture, characterized in that alumina is deposited on the surface of the magnetic powder to an amount of 0.1 to 1%." (4) Fine zirconia particles are deposited on the magnetic particle surfaces, and an oxide layer containing Al and/or Si is formed on the zirconia particle-surfaces of the magnetic particles (Japanese Patent Application Laid-Open (KOKAI) No. 1-298030). Specifically, Japanese Patent Application Laid-Open (KOKAI) No. 1-298030 discloses "Magnetic particles in which fine zirconia particles or non-magnetic fine ferric oxide particles are present between the magnetic particle surfaces and an oxide or hydroxide layer containing Al and/or Si and covering said particle surfaces" and "A process for producing magnetic particles which comprises preparing a mixed suspension containing magnetic particles and fine zirconia particles or non-magnetic fine ferric oxide particles and having a pH of not more than 4 or not less than 9, adding an acid or alkali solution thereto to adjust the pH to 6.5 to 8.5, thereby causing deposition of the said fine zirconia particles or non-magnetic fine ferric oxide particles on the surfaces of said magnetic particles, then adding an alkali solution to the suspension containing said magnetic particles having said fine zirconia particles or non-magnetic fine ferric oxide particles deposited thereon to prepare a suspension with a pH of not less than 10, adding an Al-containing compound and/or a Si-containing compound to said suspension, and further adding thereto an acid to adjust the pH to 6.5 to 8.5, thereby forming an oxide or hydroxide layer containing Al and/or Si on the surfaces of said magnetic particles having said fine zirconia particles or non-magnetic fine ferric oxide particles deposited thereon." (5) The magnetite particles of various shapes produced by a wet process are compressed by using a wheel-type kneader or an attrition mill (Japanese Patent Application Laid-Open (KOKAI) Nos. 3-131863 and 3-131865). As means for improving the properties of magnetic toner itself, the following methods are known: (6) A mixture of hydrophobic magnetite particles and non-magnetic acicular metal oxide or hydrous iron oxide particles is dispersed in a resin (Japanese Patent Application Laid-Open (KOKAI) No. 2-97968). (7) A mixture of magnetite particles, α-hematite particles and γ-hematite particles is dispersed in a resin (Japanese Patent Application Laid-Open (KOKAI) No. 63-178256). The magnetic particles with a high fluidity, a low oil absorption, an excellent charging stability and a good initial dispersibility are most strongly required at present, but such magnetic particles are still unavailable. The magnetic particles obtained from the above-described methods (1) and (3) are satisfactory in miscibility and dispersibility in resin, but are incapable of improving fluidity and charging stability to a satisfactory degree. The magnetic particles obtained from the method (2) are indeed improved in fluidity, but their oil absorption is as high as not less than 22 cc/100 g, which indicates poor miscibility and dispersibility with resins. The method (4) can not improve the initial dispersibility to a satisfactory degree. The method (5), although effective for improving the miscibility and the dispersibility, is unable to improve a fluidity and a charging stability. The method (6) is effective for stabilizing the charging amount of the toner when using hydrophobic magnetite, but this method does not improve the fluidity. In this method, the non-magnetic particles are simply mixed in a resin. The method (7) is incapable of improving the fluidity. Thus, it is strongly demanded to provide magnetic particles which has a high fluidity, a low oil absorptivity, an excellent charging stability and a good initial dispersibility. As a result of studies undertaken by the present inventors to overcome the foregoing problems of the related arts, it has been found that by reacting an aqueous ferrous salt solution and an aqueous alkali hydroxide solution of an amount of 0.90 to 0.99 equivalent to Fe 2+ in the said ferrous salt solution wherein a water-soluble silicate is previously added in an amount of 0.4 to 4.0 mol% (calculated as Si) based on Fe, to the said aqueous alkali hydroxide solution or the obtained ferrous salt reaction solution containing colloidal ferrous hydroxide, passing an oxygen-containing gas through the ferrous salt-reaction solution containing colloidal ferrous hydroxide at a temperature of 85 to 100°C to carry out the oxidation reaction, thereby producing Si-containing magnetite particles, then adding an aqueous alkali hydroxide solution in an amount of not less than 1.00 equivalent to residual Fe 2+ , to the suspension after completion of the oxidation reaction, further carrying out an oxidation reaction at 85 to 100°C to produce the spherical magnetite particles containing silicon element, then adding a water-soluble aluminum salt to the residual silicate-containing alkaline suspension wherein the said spherical magnetite particles containing silicon element have been formed, so that the amount of said aluminum salt becomes 0.01 to 2.0 wt% (calculated as Al) based on the produced particles, then adjusting tne pH of the solution to 5 to 9 to form a hydrous coprecipitate composed of silica and alumina on the surfaces of the said spherical magnetite particles containing silicon element, then filtering, washing with water and drying the precipitate, and if necessary subjecting the thus obtained dry particles to compaction, shearing and spatula-stroking by a wheel-type kneader or an attrition mill, the magnetic particles obtained have an excellent fluidity, and are low in oil absorption and also excellent in charging stability. The present invention has been achieved on the basis of these findings. It is an object of the present invention to provide magnetic particles for magnetic toner having an excellent fluidity, a low oil absorption, a high changing stability and a good initial dispersibility, and a process for industrially producing such magnetic particles. The present invention provides magnetic particles comprising magnetite particles containing in the inside thereof silicon element in an amount of 0.10 to 1.00 wt% (calculated as Si), and a hydrous coprecipitate composed of silica and alumina and deposited or adhered on the said particle surfaces, and having an area-average diameter of the said magnetic particles of 0.05 to 1.0 µm. The present invention also provides magnetic particles for magnetic toner comprising spherical magnetite particles containing in the inside thereof silicon element in an amount of 0.10 to 1.00 wt% (calculated as Si), a hydrous coprecipitate composed of silica and alumina and adhered on the spherical magnetite particle surfaces and non-magnetic fine oxides particles and/or non-magnetic fine hydrous oxides particles composed of at least one element selected from the group consisting of Fe, Ti, Zr, Si and Al and adhered on the said hydrous coprecipitate in an amount of 0.1 to 10 wt% (calculated as the element), and having an area-average particle diameter of 0.05 to 0.5 µm. The present invention also provides a process for producing the magnetic particles, which comprises reacting an aqueous ferrous salt solution and an aqueous alkali hydroxide solution of an amount of 0.90 to 0.99 equivalent to Fe 2+ in said ferrous salt solution, a water-soluble silicate being previously added in an amount of 0.4 to 4.0 mol% (calculated as Si) based on Fe, to said aqueous alkali hydroxide solution or the produced ferrous salt reaction solution containing colloidal ferrous hydroxide; passing an oxygen-containing gas through said ferrous salt reaction solution containing colloidal ferrous hydroxide under heating at 85 to 100°C to carry out an oxidation reaction, thereby producing magnetite particles containing silicon element from said colloidal ferrous hydroxide; adding an aqueous alkali hydroxide solution in an amount of not less than 1.00 equivalent to Fe 2+ remaining in the suspension after completion of the oxidation reaction; further conducting an oxidation reaction while heating at 85 to 100°C to produce the spherical magnetite particles containing silicon element; then adding a water-soluble aluminum salt to the alkaline suspension containing residual silicate, in which said spherical magnetite particles containing silicon element have been produced, so that the amount of said water-soluble aluminum salt added is 0.01 to 2.0 wt% (calculated as Al) based on the produced particles; and then adjusting the pH of the suspension to 5 to 9, thereby causing said residual silicate and said added Al to deposit as a hydrous coprecipitate of silica and alumina on the surfaces of said spherical magnetite particles containing silicon element. The present invention further provides a process for producing the magnetic particles, which comprises reacting an aqueous ferrous salt solution and an aqueous alkali hydroxide solution of an amount of 0.90 to 0.99 equivalent to Fe 2+ in said ferrous salt solution, a water-soluble silicate being previously added in an amount of 0.4 to 4.0 mol% (calculated as Si) based on Fe, to said aqueous alkali hydroxide solution or the produced ferrous salt reaction solution containing colloidal ferrous hydroxide; passing an oxygen-containing gas through said ferrous salt reaction solution containing colloidal ferrous hydroxide under heating at 85 to 100°C to carry out an oxidation reaction, thereby producing magnetite particles containing silicon element from said colloidal ferrous hydroxide; adding an aqueous alkali hydroxide solution in an amount of not less than 1.00 equivalent to Fe 2+ remaining in the suspension after completion of the oxidation reaction; further conducting an oxidation reaction while heating at 85 to 100°C to produce the spherical magnetite particles containing silicon element; then adding a water-soluble aluminum salt to the alkaline suspension containing residual silicate, in which said spherical magnetite particles containing silicon element have been produced, so that the amount of said water-soluble aluminum salt added is 0.01 to 2.0 wt% (calculated as Al) based on the produced particles; then adjusting the pH of the suspension to 5 to 9, thereby causing said residual silicate and said added Al to deposit as a hydrous coprecipitate of silica and alumina on the surfaces of said spherical magnetite particles containing silicon element; then filtering out the produced particles; washing with water and drying; and subjecting the obtained dry particles to compaction, shearing and spatula-stroking by using a wheel-type kneader or an attrition mill. The present invention also provides a process for producing the magnetic particles, which comprises reacting an aqueous ferrous salt solution and an aqueous alkali hydroxide solution of an amount of 0.90 to 0.99 equivalent to Fe 2+ in said ferrous salt solution, a water-soluble silicate being previously added in an amount of 0.4 to 4.0 mol% (calculated as Si) based on Fe, to said aqueous alkali hydroxide solution or the produced ferrous salt reaction solution containing colloidal ferrous hydroxide; passing an oxygen-containing gas through said ferrous salt reaction solution containing colloidal ferrous hydroxide under heating at 85 to 100°C to carry out an oxidation reaction, thereby producing magnetite particles containing silicon element from said colloidal ferrous hydroxide; adding an aqueous alkali hydroxide solution in an amount of not less than 1.00 equivalent to Fe 2+ remaining in the suspension after completion of the oxidation reaction; further conducting an oxidation reaction while heating at 85 to 100°C to produce the spherical magnetite particles containing silicon element; then adding a water-soluble aluminum salt to the alkaline suspension containing residual silicate, in which said spherical magnetite particles containing silicon element have been produced, so that the amount of said water-soluble aluminum salt added is 0.01 to 2.0 wt% (calculated as Al) based on the produced particles; and then adjusting the pH of the suspension to 5 to 9, thereby causing said residual silicate and said added Al to deposit as a hydrous coprecipitate of silica and alumina on the surfaces of said spherical magnetite particles containing silicon element; adding non-magnetic fine oxides particles and/or non-magnetic fine hydrous oxides particles composed of at least one element selected from the group consisting of Fe, Ti, Zr, Si and Al in an amount of 0.1 to 25 parts by weight to 100 parts by weight of the obtained spherical magnetite particles having an area-average diameter of 0.05 to 0.5 µm; and subjecting the obtained mixture to compaction, shearing and spatula-stroking by using a wheel-type kneader or an attrition mill, so that non-magnetic fine oxides particles and/or non-magnetic fine hydrous oxides particles are adhered on said silica-alumina hydrous coprecipitate. The present invention also provides a a process for producing the magnetic particles, which comprises reacting an aqueous ferrous salt solution and an aqueous alkali hydroxide solution of an amount of 0.90 to 0.99 equivalent to Fe 2+ in said ferrous salt solution, a water-soluble silicate being previously added in an amount of 0.4 to 4.0 mol% (calculated as Si) based on Fe, to said aqueous alkali hydroxide solution or the produced ferrous salt reaction solution containing colloidal ferrous hydroxide; passing an oxygen-containing gas through said ferrous salt reaction solution containing colloidal ferrous hydroxide under heating at 85 to 100°C to carry out an oxidation reaction, thereby producing magnetite particles containing silicon element from said colloidal ferrous hydroxide; adding an aqueous alkali hydroxide solution in an amount of not less than 1.00 equivalent to Fe 2+ remaining in the suspension after completion of the oxidation reaction; further conducting an oxidation reaction while heating at 85 to 100°C to produce the spherical magnetite particles containing silicon element; then adding a water-soluble aluminum salt to the alkaline suspension containing residual silicate, in which said spherical magnetite particles containing silicon element have been produced, so that the amount of said water-soluble aluminum salt added is 0.01 to 2.0 wt% (calculated as Al) based on the produced particles; and then adjusting the pH of the suspension to 5 to 9, thereby causing said residual silicate and said added Al to deposit as a hydrous coprecipitate of silica and alumina on the surfaces of said spherical magnetite particles containing silicon element; then filtering out the produced particles; washing with water and drying; subjecting the obtained dry particles to compaction, shearing and spatula-stroking by using a wheel-type kneader or an attrition mill, adding non-magnetic fine oxides particles and/or non-magnetic fine hydrous oxides particles composed of at least one element selected from the group consisting of Fe, Ti, Zr, Si and Al in an amount of 0.1 to 25 parts by weight to 100 parts by weight of the obtained spherical magnetite particles having an area-average diameter of 0.05 to 0.5 µm; and subjecting the obtained mixture to compaction, shearing and spatula-stroking by using a wheel-type kneader or an attrition mill, so that non-magnetic fine oxides particles and/or non-magnetic fine hydrous oxides particles are adhered on said silica-alumina hydrous coprecipitate. The magnetic particles according to che present invention contains in the inside thereof silicon element in an amount of 0.10 to 1.00 wt% (calculated as Si). When the content of silicon element is less than 0.10 wt%, the adhesive force of the hydrous coprecipitate composed of silica and alumina to the spherical magnetite particle surfaces is reduced. When the content thereof exceeds 1.00 wt%, the magnetic stability of the spherical magnetite particles lowers to cause distribution of coercive force and residual magnetization. The preferred range of the silicon content is 0.20 to 0.90 wt% (calculated as Si). In the hydrous coprecipitate of silica and alumina deposited or adhered on the magnetic particle surfaces in the present invention, the silica content is 0.01 to 0.5 wt% (calculated as Si) based on the final product (magnetic particles) and the alumina content is 0.01 to 2.0 wt% (calculated as Al) based on the final product (magnetic particles). It is preferred that the total amount of silica and alumina in the hydrous coprecipitate is in the range of 0.02 to 2.5 wt% (calculated as Si and Al) When the silica content in the hydrous coprecipitate is less than 0.01 wt% (calculated as Si), it is difficult to improve the fluidity and also the charging stability may be deteriorated. When the silica content exceeds 0.3 wt% (calculated as Si), although it is possible to improve the fluidity, the charging stability and the environmental stability may be deteriorated. When the alumina content is less than 0.01 wt% (calculated as Al), it is difficult to improve the charging stability, and when the alumina content exceeds 2.0 wt% (calculated as Al), the environmental stability may be deteriorated. The more preferred range of the silica content in the hydrous coprecipitate is 0.01 to 0.3 wt% (calculated as Si) based on the final product (magnetic particles), more preferred range of the alumina content in the hydrous coprecipitate is 0.02 to 1.5 wt% (calculated as Al) based on the final product (magnetic particles), and more preferred range of the total amount of silica and alumina in the hydrous coprecipitate is 0.03 to 1.8 wt% (calculated as Si and Al) based on the final product (magnetic particles). The silica to alumina ratio in the said hydrous coprecipitate, in view of a fluidity, an oil absorption and a charging stability, is preferably in the range of 10:1 to 1:100, more preferably 2:1 to 1:100, still more preferably 1:1 to 1:70 (expressed as molar ratio of Si to Al). The size of the magnetic particles according to the present invention is 0.05 to 1.0 µm (expressed as area-average particle diameter). When the particle size is less than 0.05 µm, it becomes difficult to mix and disperse the magnetite particles in resins. When the particle size exceeds 1.0 µm, although the particles are easily mixed and dispersed in resins, the magnetite particles become liable to release from the magnetic toner surface. The preferred range of the particle size is 0.05 to 0.5 µm, more preferably 0.07 to 0.4 µm (expressed as area-average particle diameter). The spherical magnetite particles before subjected to compaction treatment according to the present invention have deposited (attached) on their surfaces the hydrous coprecipitate composed of silica and alumina (hereinafter referred to as "silica-alumina hydrous coprecipitate"). The adhesion strength as determined in the manner described later, is around 90 to 100. The spherical magnetite particles after subjected to compaction treatment have the silica-alumina hydrous coprecipitate pressed to the particle surfaces so that the said hydrous coprecipitate is more strongly attached (hereinafter referred to as "adhered") to the particle surfaces. The adhesion strength is around 95 to 100. The non-magnetic fine oxides particles and/or non-magnetic fine hydrous oxides particles composed of an element selected from the group consisting of Fe, Ti, Zr, Si and Al, and adhering to the silica-alumina hydrous coprecipitate on the magnetic particle surfaces (hereinafter referred to as "non-magnetic fine oxides and/or hydrous oxides particles") include, for instance, non-magnetic fine oxides particles such as granular, acicular (columnar), spindle, or plate-like (lamellar) hematite (α-Fe 2 O 3 ) fine particles, granular or columnar TiO 2 fine particles, granular ZrO 2 fine particles, granular SiO 2 fine particles and granular or acicular Al 2 O 3 fine particles; and non-magnetic fine hydrous oxides particles such as granular, acicular (columnar), spindle, or plate-like (lamellar) hydrous ferric oxide fine particles such as goethite, lepidocrosite and akageneite fine particles, hydrous-aluminum oxide fine particles such as AlOOH fine particles, and hydrous titanium oxide fine particles such as TiOOH 2 fine particles. The size of the said non-magnetic fine oxides and hydrous oxides particles is 0.01 to 0.1 µm. When the particle size is less than 0.01 µm or exceeds 0.1 µm, the initial dispersibility tends to deteriorate. Considering the initial dispersibility, the particle size is preferably in the range of 0.02 to 0.06 µm. The size of the non-magnetic fine oxides and hydrous oxides particles of a specific element adhering to the silica-alumina hydrous coprecipitate on the magnetic particle surfaces according to the present invention is preferably the one which satisfies the following formulae (1) to (4) : 1/100 ≤ b/a ≤ 1/3 1/100 ≤ c/a ≤ 1 1/100 ≤ d/a ≤ 1/3 1/100 ≤ d/c < 1 more preferably one which satisfies the following formulae (5) to (8) : 1/50 ≤ b/a ≤ 1/5 1/50 ≤ c/a ≤ 1/2 1/50 ≤ d/a ≤ 1/5 1/10 ≤ d/c < 1 wherein a is an average particle diameter of the spherical magnetite particles as core particles, b is an average particle diameter of the granular non-magnetic fine oxides and/or hydrous oxides particles in case of granular, c is an average major axial diameter or average plate-surface diameter of the non-magnetic fine oxides and/or hydrous oxides particles in case of acicular (columnar), spindle or plate-like, and d is an average minor axial diameter or lamellar thickness of the non-magnetic fine oxides and hydrous oxides particles in case of acicular (columnar), spindle or plate-like. When the b/a ratio is less than 1/100, it is difficult to improve a dispersibility of the magnetic particles, and when the b/a ratio exceeds 1/3, it is difficult to adhere the non-magnetic fine oxides and/or hydrous oxides particles to the magnetite particle surfaces. When the c/a ratio is less than 1/100, it is difficult to improve a dispersibility of the magnetic particles, and when the c/a ratio exceeds 1, it is difficult to adhere the non-magnetic fine oxides and/or hydrous oxides particles to the magnetite particle surfaces. When the d/a ratio is less than 1/100, it is difficult to improve a dispersibility of the magnetic particles, and when the b/a ratio exceeds 1/3, it is difficult to adhere the non-magnetic fine oxides and/or hydrous oxides particles to the magnetite particle surfaces. When the d/c ratio is less than 1/100, the non-magnetic fine oxides and/or hydrous oxides particles tend to break during the adhering-treatment and the produced powder can contribute deterioration of dispersibility. The amount of the non-magnetic fine oxides and/or hydrous oxides particles of a specific element adhering to the silica-alumina hydrous coprecipitate on the magnetite particle surfaces according to the present invention is 0.1 to 10 wt% based on the final particles. When the amount of the said non-magnetic fine oxides and/or hydrous oxides particles is less than 0.1 wt% or exceeds 10 wt%, the initial dispersibility may be deteriorated. Further, when said amount of the said non-magnetic fine oxides and/or hydrous oxides particles exceeds 10 wt%, the magnetization lowers to deteriorate image quality. Considering the initial dispersibility, the said amount is preferably 0.5 to 10 wt%, more preferably 0.5 to 5.0 wt%. A process for producing the magnetic particles according to the present invention is described below. The spherical magnetite particles used in the present invention are particles represented by the formula: FeO x ·Fe 2 O 3 (O<x≤1), particles containing in the inside thereof a silicon element and assuming a black color. These particles may also contain Al, Zn, Mn, Mg and/or other elements which are usually added for improving the specified properties of the produced magnetic particles for magnetic toner, in an amount of approximately 0.01 to 1.0 wt%, preferably 0.1 to 1.0 wt%. The "spherical particles" referred to herein are particles specified by a sphericity factor of not less than 0.75, preferably 0.8 to 1.0, the sphericity factor being determined by calculating respectively the average minimal diameter and the average maximal diameter from the measured maximum diameter and minimal diameter of not less than 200 pieces of the magnetite particles shown in a transmission electron micrograph, and then dividing the average minimal diameter by the average maximal diameter. The magnetic particles (spherical magnetite particles) of the present invention containing a silicon element in the inside thereof and having a hydrous coprecipitate of silica and alumina deposited or adhered on their surfaces can be obtained from a process comprising the steps of: reacting an aqueous ferrous salt solution and an aqueous alkali hydroxide solution of an amount of 0.90 to 0.99 equivalent to Fe2+ in the said ferrous salt solution, a water-soluble silicate being previously added in an amount of 0.4 to 4.0 mol% (calculated as Si) based on Fe, to the said aqueous alkali hydroxide solution or the produced ferrous salt reaction solution containing colloidal ferrous hydroxide; passing an oxygen-containing gas through the said ferrous salt reaction solution containing colloidal ferrous hydroxide under heating at 85 to 100°C to conduct an oxidation reaction, thereby producing magnetite particles containing silicon element from the said colloidal ferrous hydroxide; then adding an aqueous alkali hydroxide solution of an amount of not less than 1.00 equivalent to Fe 2+ remaining in the suspension after completion of the oxidation reaction; further conducting an oxidation reaction under heating at 85 to 100°C to produce spherical magnetite particles containing silicon element; adding a water-soluble aluminum salt to the Si-containing alkaline suspension in which the said spherical magnetite particles containing silicon element have been produced, so that the content of the said water-soluble aluminum salt becomes 0.01 to 2.0 wt% (calculated as Al) based on the produced particles; adjusting the pH of the suspension to 5 to 9, thereby causing the said residual silicate and the said added Al to deposit or adhere as a hydrous coprecipitate composed of silica and alumina on the surfaces of the said spherical magnetite particles containing silicon element; then filtering, washing with water and drying the resulting product (magnetic particles); and if necessary subjecting the dried particles to compaction, shearing and spatula-stroking by using a wheel-type kneader or an attrition mill. In the process of the present invention, as the aqueous ferrous salt solution, there can be used an aqueous ferrous sulfate solution, an aqueous ferrous chloride solution and the like. As the aqueous alkali hydroxide solution, there can be used an aqueous solution of a hydroxide of an alkali metal such as sodium hydroxide, potassium hydroxide, etc., or an alkaline earth metal such as magnesium hydroxide, calcium hydroxide, etc. The amount of the aqueous alkali hydroxide solution used for causing precipitation of colloidal ferrous hydroxide is 0.90 to 0.99 equivalent to Fe 2+ in the aqueous ferrous salt solution. When the amount of the aqueous alkali hydroxide solution is less than 0.90 equivalent, the acicular hydrous iron oxide particles tend to form as by-products. When the amount of the aqueous alkali hydroxide solution exceeds 0.99 equivalent, it is difficult to produce the spherical magnetite particles. The oxidation reaction temperature in the present invention is 85 to 100°C. When the reaction temperature is less than 85°C, the acicular hydrous iron oxide particles other than the desired spherical magnetite particles tend to produce as by-products. When the reaction temperature is higher than 100°C, although the spherical magnetite particles can be produced, the process is industrially impractical. Oxidation is effected by passing an oxygen-containing gas (such as air) through the solution. As the water-soluble silicate, there can be used the silicates of sodium, potassium, etc. The amount of the water-soluble silicate added is 0.4 to 4.0 mol%, preferably 0.4 to 3.5 mol% (calculated as Si) based on Fe. When the amount of the water-soluble silicate is less than 0.4 mol%, it is difficult to cause coprecipitation of silica and alumina on the particle surfaces because of the insufficient amount of Si remaining in the alkaline suspension after production of the spherica magnetite particles containing silicon element. When the amount of the water-soluble silicate exceeds 4.0 mol%, the obtained spherical magnetite particles are poor in environmental stability. In the process of the present inventior, the amount of Si remaining in the alkaline suspension is not less than 0.01 wt%, preferably 0.02 to 0.4 wt% (calculated as Si), based on the final product. In the present invention, a water-soluble silicate is used for incorporating a silicon element in the spherical magnetite particles. Addition of the water-soluble silicate also contributes to improvement of the shape (spherical) of the produced particles. Concerning the time of addition of the water-soluble silicate, it is necessary to precede the production of the magnetite particles by passing of an oxygen-containing gas through the ferrous salt reaction solution containing colloidal ferrous hydroxide. The water-soluble silicate may be added either to the aqueous alkali hydroxide solution or to the ferrous salt reaction solution containing colloidal ferrous hydroxide. When the water-soluble silicate is added to the ferrous salt solution, the silicate is precipitated as SiO 2 immediately after addition of the water-soluble silicate, so that it is impossible to uniformly contain silicon element in the inside of the spherical magnetite particles and the object of the present invention can not be attained. The amount of the aqueous alkali hydroxide solution used in the present invention is not less than 1.00 equivalent to residual Fe 2+ . When the amount of the aqueous alkali hydroxide solution is less than 1.00 equivalent, all of Fe 2+ is not precipitated. In view of the industrialization, the amount of 1.00 to 2.00 equivalent to the residual Fe 2+ is preferred. The temperature of the alkaline suspension at the time of addition of the water-soluble aluminum salt thereto is 50 to 100°C. When the said temperature of the alkaline suspension is less than 50°C, the magnetite particles are not well dispersed in the suspension, hindering uniform coprecipitation of silica and alumina. When the temperature of the said alkaline solution is higher than 100°C, although it is possible to maintain uniform dispersion of the spherical magnetite particles in the suspension, the process is not economical. Oxidation is effected by passing an oxygen-containing gas (such as air) through the solution. As the water-soluble aluminum salt used in the present invention, aluminum sulfate, aluminum nitrate and aluminum chloride can be exemplified. The amount of the water-soluble aluminum salt used in the process of the present invention is 0.01 to 2.0 wt%, preferably 0.01 to 1.5 wt% (calculated as Al) based on the produced particle. When the said amount of the water-soluble aluminum salt is less than 0.01 wt%, it is difficult to precipitate an enough amount of Al to impart a desired improvement of charging stability. When the said amount of the water-soluble aluminum salt exceeds 2.0 wt%, an excessive amount of Al is deposited (attached) on the magnetite particles, thereby reducing their environmental stability. Considering the environmental stability of the produced particles, the amount of the said water-soluble aluminum salt to be added is preferably 0.01 to 1.5 wt%. In the process of the present invention, the pH of the alkaline suspension after addition of the water-soluble aluminum salt is adjusted to 5 to 9. When the pH value of the alkaline suspension is less than 5 or greater than 9, the silica-alumina hydrous coprecipitate can not be readily deposited (attached) on the magnetite particle surfaces. A wheel-type kneader or an attrition mill can be used for the compaction of the magnetite particles having the silica-alumina hydrous coprecipitate deposited on the surfaces. The wheel-type kneaders usable in the present invention include Simpson mix muller, multimill, Stotz mill, back flow mixer, Irich mill, etc. Wet pan mill, melanger, whirl mill and quick mill can not be used in the present invention since they merely have the functions of compaction and spatula-stroking, and no shearing action. Deposition (attachment) of the non-magnetic fine oxides and/or hydrous oxides particles composed of a specific element in the present invention can be accomplished by adding and mixing the non-magnetic fine oxides and/or hydrous oxides particles in the suspension containing magnetic particles (spherical magnetite particles) after having the silica-alumina hydrous coprecipitate deposited (attached) on the spherical magnetite particle surfaces, and then subjecting the suspension to filtration, water-washing and drying; or by adding the non-magnetic fine oxides and/or hydrous oxides particles to the magnetic particles (spherical magnetite particles) containing a silicon element in the inside thereof and having the silica-alumina hydrous coprecipitate on the surfaces, which have been obtained after filtration, water-washing and drying; and then subjecting the said particles to dry-mixing. The amount of the non-magnetic fine oxides and/or hydrous oxides particles composed of a specific element is not more than 12 parts by weight, preferably 0.1 to 12 parts by weight, more preferably 0.1 to 10 parts by weight based on 100 parts by weight of the particles to be treated. When the said amount is less than 0.1 part by weight, the obtained particles may show no significant improvement of the fluidity. When the said amount exceeds 12 parts by weight, the magnetization of the particles lowers to deteriorate the image quality. Adhering-treatment according to the present invention can be conducted, for example, by compressing, shearing and spatula-stroking the magnetic particles (spherical magnetic particles) containing silicon in the inside and having the silica-alumina hydrous coprecipitate deposited on the surfaces, and the non-magnetic fine oxides and/or hydrous oxides particles of a specific element by using a wheel-type kneader or an attrition mill. As the wheel-type kneader, there can be used Simpson mix muller, multimill, back flow mixer, Irich mill, etc., but wet pan mill, melanger, whirl mill and quick mill are inapplicable since they merely perform compaction and spatula-stroking and no shearing work. In case of using a wheel-type kneader for the said adhering-treatment, the linear load is preferably in the range of 10 to 200 kg/cm. When the linear load is less than 10 kg/cm, it is difficult to adhere the non-magnetic fine oxides and/or hydrous oxides particles to the magnetic particles. When the linear load is greater than 200 kg/cm, the particles may be broken. The more preferred range of the linear load is 20 to 150 kg/cm. In case the said adhering-treatment is carried out by using a wheel-type kneader, the treating time is 10 to 120 minutes. When the treating time is less than 10 minutes, it is difficult to adhere the non-magnetic fine oxides and/or hydrous oxides particles to the magnetic particles. When the treating time exceeds 120 minutes, it is unfavorable in terms of economy although the desired adhering-treatment can be accomplished. The more preferred range of treating time is 20 to 90 minutes. One of the most important points of the present invention is the fact that by adding a water-soluble aluminum salt to an alkaline suspension containing residual silicate, in which the spherical magnetite particles containing silicon element have been produced, and adjusting the pH of the solution to 5 to 9, there can be obtained the spherical magnetite particles containing in the inside thereof silicon element in an amount of 0.10 to 1.00 wt% (calculated as Si) and also having a hydrous coprecipitate of silica and alumina deposited on their surfaces and having an area-average diameter of 0.05 to 0.5 µm; and these spherical magnetite particles have a high fluidity, a low oil absorption and an excellent charging stability, and the fact that when the non-magnetic fine oxides and/or hydrous oxides particles of a specific element are further adhered to the said silica-alumina bydrous coprecipitate, the obtained particles are improved in initial dispersibility while maintaining said properties such as a high fluidity, a low oil absorption and an excellent charging stability. The present inventors consider that these specificities may be attributed to the following fact. Since the spherical magnetite particles according to the present invention have a low oil absorption as shown in the Examples and the Comparative Examples described later, these particles have little tendency to agglomerate each other and therefore, their surfaces can be wetted sufficiently with a small amount of linseed oil. This accounts for an excellent dispersibility of the particles of the present invention. Regarding the reason why the magnetic particles according to the present invention have little tendency to agglomerate each other, the present inventors refer to the following facts. Since the hydrous coprecipitate composed of silica and alumina is produced in a solution, such hydrous coprecipitate is fine and uniform in particle size, and also it has good adaptability to the magnetic particles since silicon element is contained in the inside of the spherical magnetite particles. Further, since the hydrous coprecipitate contains aluminum element which has a higher affinity for Fe than silicon element, the said hydrous coprecipitate tends to deposit on the magnetite particle surfaces, and as a result, the fine and uniform silica-alumina hydrous coprecipitate particles are evenly distributed on the surfaces of the individual magnetic particles existing independently of each other in the solution. For this reason probably, the magnetic particles of the present invention have little tendency to agglomerate each other even after subjected to such treatments as filtration, washing with water and drying. Excellent fluidity of the spherical magnetite particles according to the present invention may be accounted for by the fact that since a large number of the fine and uniform silica-alumina hydrous coprecipitate particles are uniformly distributed on the magnetite particle surfaces as mentioned above, there is provided a sufficient unevenness of the particle surface and also the individual magnetite particles are sufficiently spaced apart from each other to discourage magnetic agglomeration of the particles. Excellent charging stability of the spherical magnetite particles according to the present invention may be ascribed to the fact that since a large number of the fine and uniform silica-alumina hydrous coprecipitate particles are evenly distributed on the magnetite particle surfaces as mentioned above, the negative charges of silicon element and the positive charges of aluminum element disposed between the silicon elements counteract each other to even off the charges of both polarities. As regards an excellent initial dispersibility imparted to the spherical magnetite particles of the present invention while maintaining the above-mentioned characteristics, the present inventors consider that as noted from a scanning electron micrographical observation, the non-magnetic fine oxides and/or hydrous oxides particles are particles whose the minimal diameter is greater than the height of the convexities of unevenness formed by the silica-alumina hydrous coprecipitate and which also have an affinity for the said hydrous coprecipitate, and further the said non-magnetic fine oxides and/or hydrous oxides particles are adhered to the silica-alumina hydrous coprecipitate on the spherical magnetite particles in a smaller amount than required for entirely covering the surfaces of the spherical magnetite particles having the silica-alumina hydrous coprecipitate, so that the said non-magnetic fine oxides and/or hydrous oxides particles are strongly adhered to the silica-alumina hydrous coprecipitate of the magnetite particle, with the individual non-magnetic fine oxides and/or hydrous oxides particles being properly spaced apart from each other and smashing down the convexities of unevenness formed by the silica-alumina hydrous coprecipitate, with the lower portions of the said non-magnetic fine oxides and/or hydrous oxides particles in the thickness direction being embedded in the silica-alumina hydrous coprecipitate of the magnetic particles. Consequently, the portions of the spherical magnetite particle surfaces where unevenness formed by the hydrous coprecipitate is maintained contributes to the improvement of the properties such as a fluidity, an oil absorption and a charging stability. On the other hand, since the height of the non-magnetic fine oxides and/or hydrous oxides particles at the portions of the spherical magnetite particles where the non-magnetic fine oxides and/or hydrous oxides particles are adhered, is greater than that of the convexities of unevenness formed by the hydrous coprecipitate, the individual spherical magnetite particles can be spaced apart wider from each other and the magnetic agglomerative tendency of the particles is further weakened, resulting in a reduced Van der Waals force. The granular magnetite particles before comp ction treatment according to the present invention have a compaction degree of not more than 60%, an oil absorption of not more than 20 ml/100 g, a charging stability of not more than 1.0µc/g, a charging amount of not less than -50 µc/g, an initial dispersibility of not more than 1.3, a bulk density of 0.4 to 1.0 g/cc and a tap density of 0.6 to 1.5 g/cc. Concerning a reason why the fluidity is enhanced and the oil absorption is reduced after the compaction treatment according to the present invention, the present inventors consider that not only the silica-alumina hydrous coprecipitate particles which are deposited (attached) on the magnetite particle surfaces but also the silica-alumina hydrous coprecipitate particles which are merely held between the magnetite particles, are equally pressed against the magnetite particle surfaces in the compaction treatment, thereby causing a greater number of the hydrous coprecipitate particles to be more adhered to the magnetite particle surfaces and to be kept from releasing, so that the effects of the said silica-alumina hydrous coprecipitate are full exhibited. Further, regarding the reason why the charging stability of the magnetite particles is improved after compaction treatment according to the present invention, the present inventors consider that the silica-alumina hydrous coprecipitate particles held between the magnetite particles are also forced to adhere to the magnetite particle surfaces by compaction treatment to promote leveling of charges. The spherical magnetite particles having the said non-magnetic fine oxides and/or hydrous oxides particles deposited on the silica-alumina hydrous coprecipitate according to the present invention have a compaction degree of not more than 40%, an oil absorption of not more than 18 ml/100 g, a charging stability of not more than 0.8 µC/g, and an initial dispersion of not less than 1.4. Also, the said spherical magnetite particles according to the present invention have an adhesive strength of not less than 50, a magnetization of not less than 75 emu/g, a residual magnetization of not less than 2.0 emu/g, a bulk density of 0.8 to 1.3 g/cc, a tap density is 1.0 to 1.8 g/cc, and a charging amount of not less than -50 µc/g. The magnetic particles of the present invention are useful for magnetic toner. Magnetic toner produced from the magnetic particles of the present invention is obtained by mixing the particles with a resin. The resin used in the present invention is not restricted, and known binder resins for magnetic toner are usable. Examples of such resins are styrene-acrylate copolymer, styrene-butyl acrylate copolymer, polystyrene, polyvinyl chloride, phenol resin, epoxy resin, polyacrylate, polyester, polyethylene and polypropylene. The mixing ratio of the resin is 100 to 900 parts by weight, preferably 100 to 400 parts by weight, based on 100 parts by weight of the magnetic particles. The magnetic toner of the present invention may contain coloring agent, plasticizer, surface lubricant, antistatic agent, etc., in the range which does not deteriorate the dispersibility of the magnetic particles in the binder resin. A low-molecular resin such-as polyethylene or polypropylene may be added, if necessary, as an additive. In producing the magnetic toner of the present invention, known methods (e.g., a method disclosed in Japanese Patent Application Laid-Open (KOKAI) No. 2-80 (1990) corresponding to U.S. Patent No. 5,066,558 and Japanese Patent Application Laid-Open (KOKAI) No. 2-181757 (1990)) may be adopted. The particle diameter of the magnetic toner of the present invention is 3 to 15 µm, preferably 5 to 12 µm. The magnetic particles for the magnetic toner according to the present invention have a high fluidity, a low oil absorption, an excellent charging stability and a good initial dispersibility, so that they are suited for use as magnetic particles for magnetic toner. EXAMPLES: The present invention is further described with reference to the following examples and comparative examples. In the Examples and Comparative Examples , the shape of the magnetite particles was observed through a transmission electron microscope and a scanning electron microscope. The size of the magnetite particles is the area average diameter calculated from the Martin diameter (the length of the segment bisecting the projected area in a predetermined direction) of a projected particle diameter. The whole amount of Si and the whole amount of Al in a magnetite particle were determined by a fluorescent X-ray analysis carried out according to "General Rules on Fluorescent X-ray Analyses" of JIS-K-0119 by using a fluorescent X-ray analyzer Model 3063-M (manufactured by Rigaku Denki Kogyo Co., Ltd). The amount of Si attached or adhered on the magnetite particle surfaces was determined by measuring the whole amount of Si and the amount of Si contained in the particle by a fluorescent X-ray analysis according to the "General Rules on Fluorescent X-ray Analyses" of JIS-K-0119 by using a fluorescent X-ray analyzer Model 3063-M (manufactured by Rigaku Denki Kogyo Co., Ltd), and subtracting the amount of Si contained in the particle from the whole amount of Si, by following the steps (1) - (8) described below. The amount of Al existing on the magnetite particle surface was determined in the same way as used for determination of the amount of Si described above. (1) The whole amount of Si (or Al) in the produced magnetite particles is determined by the fluorescent X-ray analyzer. (2) 300 ml of a 1N NaOH solution is added to 15 g of magnetite particles and the particles are dispersed by applying ultrasonic waves for 10 minutes. (3) The dispersion is stirred at 50°C for 30 minutes. (4) The stirred dispersion is centrifuged at 10,000 r.p.m. for 10 minutes and the supernatant is separated. (5) After separation of the supernatant, a 1N NaOH solution is added to the residual solid matter, followed by ultrasonic dispersion for 5 minutes, then the solution is centrifuged for 10 minutes, and the supernatant is separated. (6) After separation of the supernatant, ion exchange matter is added to residual solid matter, followed by 5-minute ultrasonic dispersion and centrifugation. (7) The supernatant is removed and the residual solid matter is dried. (8) The amount of Si (or Al) in the dried product obtained in the step (7) is determined by the fluorescent X-ray analyzer. The thus determined amount of Si (or Al) is represented as the amount of Si (or Al) contained in the magnetite particle. The whole amounts of Fe, Ti and Zr in the magnetite particles were determined in the same way as above, by carrying out a fluorescent X-ray analysis according to the "General Rules on Fluorescent X-ray Analyses" of JIS-K-0119 using a fluorescent X-ray analyzer Model 3063-M (Rigaku Denki Kogyo Co., Ltd). The amount of Fe adhered on the magnetite particle surfaces was determined by measuring the whole amount of Fe and the amount of Fe contained in the particle, and subtracting the amount of Fe contained in the particle from the overall amount of Fe, by following the steps (a) - (g) described below. The amounts of Ti and Zr adhered on the magnetite particle were determined in the same way as the determination method of the amount of Fe described above. (a) The whole amount of Fe (or Ti or Zr) in the produced magnetite particles is determined by the fluorescent X-ray analyzer. The determined amount is expressed as Ib. (b) 50 g of sample particles are suspended in 1 litre of ion-exchanged water and treated by an ultrasonic cleaner for 60 minutes. (c) The spinel-type iron oxide particles are magnetically separated from the non-magnetic fine iron oxide and/or hydrous iron oxide particles. (d) After removing the supernatant, 1 litre of ion-exchanged water is supplied and the solution is treated by the ultrasonic cleaner for 60 minutes. (e) After repeating the above operation three times, the supernatant is removed and the residue is dried to obtain a powder. The weight of the sample at this point is measured. The measured value is expressed as X (g). (f) After ultrasonic cleaning, the whole amount of Fe (or Ti or Zr) in the sample is determined by the fluorescent X-ray analyzer. The determined value is expressed as Ia. (g) The amount of the non-magnetic fine oxides and/or hydrous oxides particles on the magnetite particle surfaces was determined from the following formula: Is = Ib - Ia × (X/50) Charging stability of the magnetite particles was determined by repeating 5 times the operation of measuring the amount of charges generated by rubbing the magnetite particles with an iron powder carrier TEFV-200/300 (produced by Nippon Teppun Co., Ltd.) for 30 minutes, by using a blow-off charge meter (manufactured by Toshiba Chemical Co., Ltd.), and expressing the difference between the maximal value and the minimal value of the five measurements as ΔQ. The smaller the ΔQ value, the better the charging stability. For determination of fluidity of the magnetite particles, in view of the close relation between fluidity and compaction degree, the tap density (ρt) and the bulk density (ρa) of the particles were measured, and the obtained values were introduced into the following formula. The produced value represents fluidity of the particles. Compaction degree = [(ρt - ρa)/(ρt)] × 100 The smaller the compaction degree, the better is fluidity. Bulk density was measured according to the pigment testing method of JIS-X-5101. Tap density was determined as follows. 10 g of the magnetite particles whose bulk density had been measured was filled gently in a 20 cc graduated cylinder by using a funnel and the packed cylinder was dropped from a height of 25 mm. After repeating this dropping 600 times, the volume (cc) of the magnetite particles in the cylinder was read from the graduations on the cylinder and the obtained value was introduced into the following formula. Tap density (g/cc) = 10 (g)/volume (cc) Oil absorption of the magnetite particles was determined from the pigment testing method of JIS-K-5101. Initial dispersibility of the magnetite particles was determined in the following way. To 0.5 g of sample particles is added 0.5 ml of castor oil, and the obtained mixture is kneaded by a Huber muller (one turn). The particle size at this point is measured by a grind gage and the obtained value is represented by  1 . Then 0.5 ml of castor oil is added to 0.5 g of sample and the mixture is kneaded by the Huber muller (5 turns). After repeating this operation twice, the particle size is measured by the grind gage. The obtained value is represented by  10 . The quotient of  1 / 10 is defined as initial dispersibility. Moisture absorption was determined as follows. The magnetite particles are deaerated at 120°C for 2 hours by a deaerator BERSORP 18 (manufactured by Japan Bell Corp). The water-vapor adsorption isotherm is measured at the adsorption temperature of 25°C and the value obtained under the relative pressure of 0.6 is defined as an index of moisture absorption. The greater the value, the higher is moisture absorption and the worse is environmental stability. The adhesion strength of the non-magnetic fine oxides and/or hydrous oxides particles of a specific element adhered to the magnetite particle surfaces was determined by the following method (I) or (II): (I) 50 g of sample magnetite particles are put into a beaker and suspended in 1 litre of ion-exchanged water. The suspension is treated in an ultrasonic cleaner for one minute and the magnetite particles are precipitated. After removing the supernatant, the magnetite particles are filtered out. The obtained solid is washed with ion exchange water and then dried. The weight of the solid after drying is expressed as Y (g). The amount (I BS ) of Fe, Ti, Zr, Si or Al before ultrasonic cleaning and the amount (I AS ) of Fe, Ti, Zr, Si or Al after cleaning are measured by fluorescent X-ray analyzer, and the obtained values are introduced into the following equation. I R = I s - {I BS - I AS x (Y/100) } Adhesive strength = (I R /I s ) x 100 (II) 10 g of sample magnetite particles are put into a beaker and suspended in 200 ml of ion exchange water. The suspension is treated in an ultrasonic cleaner for one minute and the magnetite particles are precipitated. After removing the supernatant, the magnetite particles are filtered out. The obtained solid is washed with ion exchange water and then dried. The total amount (I BS1 ) of Si and the amount of Al before ultra-sonic cleaning and the amount of Si and the total amount (I AS1 ) of Al after cleaning are measured by a fluorescent X-ray analyzer, and the measured values are introduced into the following formula: Adhesive strength = (I AS1 /I BS1 ) × 100 Specific surface area was determined according to the BET one-point method by using Mono Sorb (manufactured. by Yuasa Ionics Co., Ltd). 〈Preparation of magnetite particles〉 Example 1 To 31.2 litres of a 1.83 mol/l NaOH solution containing 2,770 g of sodium silicate (#3) (SiO 2 : 29 wt%), 18.8 litres of a 1.6 N ferrous sulfate solution was added to obtain an aqueous ferrous sulfate solution including Fe(OH) 2 . The alkali to iron ratio (20H/Fe) in this solution was 0.95, and the concentration of Fe 2 + (including Fe(OH) 2 ) was 0.6 mol/l. Air was passed at a rate of 100 litre/min, through the ferrous sulfate solution containing Fe(OH) 2 at 90°C for 120 minutes to carry out the reaction for forming an aqueous solution containing the magnetite particles. Then 1.58 litres of 6N NaOH solution (corresponding to 1.10 equivalent to Fe 2 +) was added and air was passed through the solution at a rate of 100 litre/min at 90°C for 60 minutes to carry out the reaction for forming the magnetite particles. A part of the alkaline suspension containing the magnetite particles was collected and analyzed by a plasma emission spectroscopic analyzer to confirm the presence of Si in the suspension. To this alkaline suspension containing the magnetite particles, 1.56 litres of a 10% aqueous solution of aluminum sulfate (corresponding to 0.1 wt% based on magnetite) was added and stirred for 30 minutes. Thereafter, 3N dilute sulfuric acid was added to the suspension to adjust its pH to 7. The resultantly formed black precipitate was filtered, washed with water and dried in the usual ways to obtain the black particles. The result of electron micrographic observation of these black particles showed that they were spherical. The properties of the obtained black particles are shown in Table 2. In view of the facts that a water-soluble silicate and an aluminum compound are allowed to exist at the same time in the solution, and that the obtained magnetite particles have very excellent charging stability, and silica and alumina are uniformly distributed to level off the charges as compared with the magnetite particles obtained in Comparative Example 5 shown later in which the fine silica particles and the fine alumina particles are deposited in the form of a mixture on the magnetite particle surfaces, it is considered that the magnetite particles according to the present invention have a hydrous coprecipitate of silica and alumina deposited (attached) thereon. Examples 2-4 and Comparative Examples 1-4 The procedure of Example 1 was carried out except for change of the concentration of ferrous hydroxide, the kind and amount added of silicate, the equivalent ratio of alkali added, the reaction temperature, and the kind and amount added of Al compound. The main preparation conditions used here are shown in Table 1 and the properties of the obtained magnetite particles are shown in Table 2. The magnetite particles obtained in Examples 2-4 and Comparative Examples 1-4 were all found to have a spherical shape as a result of electron microscopical observation of these particles. As shown in Table 2, the magnetite particles obtained in Comparative Example 1 had silicon alone deposited (as a result of precipitation) on their surfaces. These particles were unsatisfactory in fluidity and showed high oil absorption and poor charging stability. The magnetite particles obtained in Comparative Example 2 were high in hygroscopity and hence poor in environmental stability because of high Al content in the silica-alumina hydrous coprecipitate on the particle surfaces. The magnetite particles obtained in Comparative Example 3 had aluminum alone deposited (as a result of Precipitation) on the surfaces and were poor in fluidity. The magnetite particles obtained in Comparative Example 4 had high hygroscopicity and Low environmental stability because of high Si content in the silica-alumina hydrous coprecipitate on the particle surfaces. Comparative Example 5 To an alkaline suspension having formed therein the silica-containing spherical magnetite particles obtained in the same way as Example 3, 3N sulfuric acid was added to adjust the pH of the suspension to 7 and the whole amount of silica existing in the solution was deposited on the magnetite particle surfaces. Then, to this alkaline suspension was added the same amount of an aluminum sulfate solution as used in Example 3 while maintaining the pH at 7, thereby producing the magnetite particles having the fine silica particles and fine alumina particles deposited in a mixed state on the surfaces. The properties of the obtained magnetite particles are shown in Table 2. As shown in Table 2, these particles were poor in fluidity and charging stability as compared with the particles provided according to the present invention. Examples 5-8 2 kg of the magnetite particles obtained in Examples 1-4 were subjected to 30-minute compaction by a wheel-type kneader Sandmill MPUV-2 (manufactured by Matsumoto Chuzo Tekko Co., Ltd). The properties of the compacted magnetite particles are shown in Table 2. Examples 9-12 The same procedure as Example 1 was carried out except for change of the concentration of Fe(OH) 2 , the kind and amount added of silicate, the equivalent ratio of alkali, the reaction temperature, and the kind and amount added of Al compound. The main preparation conditions used in the procedure are shown in Table 3, and the properties of the obtained magnetite particles are shown in Table 4. The magnetite particles obtained in Examples 9-12 were confirmed to be the spherical particles as a result of electron micrographic observation. 〈Adhesion Treatment〉 Example 13 10 kg of the magnetite particles obtained in Example 1 and 300 g of fine granular TiO 2 particles having a diameter of 0.04 µm were mixed and the obtained mixture was treated in a Simpson mix muller under a linear load of 50 kg for 30 minutes to adhere the fine TiO 2 particles to the magnetite particles. Scanning electron micrographic observation of the obtained particles showed that the fine granular TiO 2 particles were adhered with proper spaces from each other on the surfaces of the spherical magnetite particles. The properties of the obtained particles are shown in Table 6. Examples 14-21 The same procedure as Example 13 was carried out except for change of the kind and amount of the magnetite particles to be treated and the non-magnetic fine oxides or hydrous oxides particles, and the adhering conditions. Scanning electron micrographical observation showed that the particles obtained in Examples 14-21 were all the spherical magnetite particles having the non-magnetic fine oxides or hydrous oxides particles adhered on the surfaces with proper spaces from each other. The properties o the obtained particles are shown in Table 6. Comparative Example 6 The same procedure as Example 13 was carried out except that TiO 2 was mixed in an amount of 2 kg. The obtained particles were low in adhesive strength and showed a high oil absorption, bad a charging stability and a poor initial dispersibility. Also, they assumed a grayish tint unfavorable for the magnetic particles to be used for magnetic toner. Comparative Example 7 The procedure of Example 15 was followed except that the amount of ZrO 2 was changed to 2 g. The obtained particles were poor in dispersibility and also unsatisfactory in initial dispersibility. Comparative Example 8 The same procedure as Example 16 was carried out except for use of the α-Fe 2 0 3 particles with a size of 0.3 µm. The obtained particles were low in adhesive strength and poor in initial dispersibility. Comparative Example 9 The same procedure as Example 16 was carried out except that the amount of the non-magnetic oxide was changed to 1 g. The obtained particles were poor in initial dispersibility. Comparative Example 10 The same procedure as Example 18 was carried out except that the amount of the non-magnetic oxide was changed to 2 g. The obtained particles were poor in initial dispersibility. Comparative Example 11 15 kg of the particles to be treated were suspended in 200 litres of an aqueous alkaline solution. To this suspension was added 259 g of #3 water glass, followed by one-hour stirring at 60°C with the pH of 8. Thereafter, the suspension was washed with water and dried to obtain dry particles. Electron micrographic observation of the obtained particles showed that no granular silica was present on the particle surfaces, indicating that silica had formed a stratiform structure. By using 10 kg of these particles, the same procedure as Example 19 was carried out except that no SiO 2 particles were used. The obtained particles had a high compaction degree and a poor initial dispersibility. Comparative Example 12 The same procedure as Example 17 was carried out except that a Henschel mixer was used as kneader. The obtained particles, due to their weak adhesive strength to the magnetite particle surfaces, had a high compaction degree, high oil absorption and poor initial dispersibility. The treating conditions used in the preparation of the treated magnetite particles obtained in Examples 13-21 and Comparative Examples 6-12 are shown in Table 5, and the properties of these particles are shown in Table 6. Referential Examples 1-11 The particles obtained in Examples 1-4 and Examples 9 to 12 and Comparative Examples 1-3 were treated by using a Simpson mix muller without adding the fine particles of non-magnetic iron oxide or fine particles of non-magnetic hydrous iron oxide. The treating conditions and the properties of the obtained treated particles are shown in Table 7.

Claims: Magnetic particles comprising magnetite particles containing in the inside thereof elemental silicon in an amount of 0.10 to 1.00 wt% (calculate as Si) and having a hydrous coprecipitate of silica and alumina on said magnetite particle surfaces, and having an area-average diameter of 0.05 to 1.0 µm. Magnetic particles according to claim 1, wherein the magnetite particles as core particles are spherical and the magnetic particles are spherical. Magnetic particles according to claim 1 or 2, wherein the respective amounts of silica and alumina in the hydrous coprecipitate are from 0.01 to 0.5 wt% (calculated as Si) and from 0.01 to 2.0 wt% (calculated as Al) based on the magnetic particles. Magnetic particles according to any one of the preceding claims, wherein the ratio (molar ratio) of silica: alumina in the hydrous coprecipitate is from 10:1 to 1:100. Magnetic particles according to any one of the preceding claims, having a compaction degree of not more than 40%, an oil absorption of not more than 18 ml/100 g and a charging stability of not more than 0.8 µC/g. Magnetic particles according to any one of the preceding claims, wherein non-magnetic fine oxide particles and/or non-magnetic fine hydrous oxide particles adhere to the hydrous coprecipitate in an amount of from 0.1 to 10 wt% based on the magnetic particles. Magnetic particles according to claim 6, wherein the non-magnetic fine oxide particles and non-magnetic fine hydrous oxide particles are composed of at least one element selected from Fe, Ti, Zr, Si and Al. Magnetic particles according to claim 7, wherein the non-magnetic fine oxide particles are granular, acicular, spindle or plate-like hematite fine particles, granular or columnar TiO 2 fine particles, granular SiO 2 fine particles or granular or acicular Al 2 O 3 fine particles; and the non-magnetic fine hydrous oxide particles are granular, acicular, spindle or plate-like goethite, lepidocrocite or akageneite fine particles, granular AlOOH fine particles or granular TiOOH 2 fine particles. Magnetic particles according to any one of claims 6 to 8, wherein the average diameter of the non-magnetic fine hydrous oxide particles is from 0.01 to 0.1 µm. Magnetic particles according to any one of claims 6 to 9, wherein the non-magnetic fine oxide particles and non-magnetic fine hydrous oxide particles are particles which satisfy the following formulae: in the case when the particles are granular: 1/100 ≤ b/a 1/3, and in the case when the particles are acicular, spindle or plate-like: 1/100 ≤ c/a ≤ 1 1/100 ≤ d/a ≤ 1/3 1/100 ≤ d/c < 1 wherein a is the average particle diameter of the magnetite particles, b is the average particle diameter of the non-magnetic fine oxide particles and non-magnetic fine hydrous oxide particles, c is the average major axial diameter or average plate-surface diameter of the non-magnetic fine oxide particles and non-magnetic fine hydrous oxide particles and d is the average minor axial diameter of the non-magnetic fine oxide particles and non-magnetic fine hydrous oxide particles or, when the particles are plate-like, the average plate thickness. Magnetic particles according to any one of claims 6 to 10 having an initial dispersibility in resin of not less than 1.4. A process for producing the magnetic particles defined in claim 1, which process comprises: (1) reacting an aqueous ferrous salt solution with from 0.90 to 0.99 equivalent, with respect to the Fe 2+ in said ferrous salt solution, of an aqueous alkali hydroxide solution to form thereby a ferrous salt reaction solution containing colloidal ferrous hydroxide, a water soluble silicate having been previously added in an amount of from 0.4 to 4.0 mol% (calculated as Si and based on Fe) to said aqueous alkali hydroxide solution or being added in said amount to said ferrous salt reaction solution; (2) passing a molecular oxygen-containing gas through said ferrous salt reaction solution containing colloidal ferrous hydroxide under heating at from 85 to 100°C to carry out an oxidation reaction and to form thereby the magnetite particles containing elemental silicon from said colloidal ferrous hydroxide; (3) adding an aqueous alkali hydroxide solution in an amount of not less than 1.00 equivalent with respect to the Fe 2+ remaining in the suspension after completion of the oxidation reaction; (4) further conducting an oxidation reaction while heating at from 85 to 100°C to produce thereby the spherical magnetite particles containing elemental silicon; (5) adding to the alkaline suspension which contains residual silicate and in which said spherical magnetite particles containing elemental silicon have been produced from 0.01 to 2.0 wt% (calculated as Al and based on the produced particles) of a water-soluble aluminium salt; and (6) adjusting the pH of the suspension to from 5 to 9 to cause thereby said residual silicate and said added Al to coprecipitate as a hydrous coprecipitate of silica and alumina on the surfaces of said spherical magnetite particles containing elemental silicon. A process according to claim 12, which further comprises: (7) filtering out, washing with water and drying the thus produced spherical magnetite particles containing elemental silicon and having the hydrous coprecipitate of silica and alumina on the surfaces thereof; and (8) subjecting the dry particles to compaction, shearing and spatula-stroking by using a wheel-type kneader or an attrition mill. A process for producing the magnetic particles defined in claim 6, which process comprises adding non-magnetic fine oxide particles and/or non-magnetic fine hydrous oxide particles composed of at least one element selected from Fe, Ti, Zr, Si and Al in an amount of from 0.1 to 25 parts by weight to 100 parts by weight of the spherical magnetite particles having an area-average diameter of from 0.05 to 0.5 µm produced by the process of claim 12 or 13, said spherical magnetite particles containing in the inside thereof from 0.10 to 1.00 wt% of elemental silicon and having a hydrous coprecipitate of silica and alumina deposited on the surfaces thereof; and subjecting the obtained mixture to compaction, shearing and spatula-stroking by using a wheel-type kneader or an attrition mill so that the non-magnetic fine oxide particles and/or non-magnetic fine hydrous oxide particles are caused to adhere onto said hydrous coprecipitate composed of silica and alumina. A magnetic toner containing magnetic particles as defined in any one of claims 1 to 11 or a coating material or moulded resin article containing, as a pigment, magnetic particles as defined in any one of claims 1 to 11.

Inventors: AOKI KOSO, FUJIOKA KAZUO, KOZAWA MINORU, MISAWA HIROMITSU, OKANO YOJI, UCHIDA NAOKI, AOKI, KOSO, FUJIOKA, KAZUO, KOZAWA, MINORU, MISAWA, HIROMITSU, OKANO, YOJI, UCHIDA, NAOKI
Applicants: TODA KOGYO CORP, TODA KOGYO CORP.
Application year: 1994
Main classification: G03G9
IPC codes: C09C1, G11B5, G03G9, H01F1